This invention relates to an oxidation and/or ammoxidation catalyst containing the elements molybdenum, bismuth, one or more elements selected from iron, nickel and cobalt, and, optionally phosphorus and an alkali metal selected from sodium and potassium.
It is well known that olefins can be oxidized to oxygenated hydrocarbons such as unsaturated aldehydes and acids, for example, acrolein and methacrolein, and acrylic and methacrylic acid. It is also well known that olefins can be ammoxidized to unsaturated nitriles such as acrylonitrile and methacrylonitrile. The value of such oxygenated hydrocarbons and unsaturated nitriles is generally well recognized with acrylonitrile being among the most valuable monomers available to the polymer industry for producing useful polymeric products.
Various catalytic processes are known for the oxidation and/or ammoxidation of olefins. Such processes commonly react an olefin or an olefin-ammonia mixture with oxygen in the vapor phase in the presence of a catalyst. For the production of acrolein and acrylonitrile, propylene is the generally used olefin reactant and for the production of methacrolein and methacrylonitrile, isobutylene is the generally used olefin reactant.
Catalysts disclosed as having significant utility in such (amm)oxidation processes are described in U.S. Pat. No. 3,882,159 and Example III of U.S. Pat. No. 3,746,657 describes such a catalyst containing the elements molybdenum, potassium, phosphorus, cobalt, iron, nickel, bismuth and oxygen deposited on a silica substrate.
A useful process by which this catalyst (and other similar catalysts) can be prepared is set forth in Example III of U.S. Pat. No. 3,746,657. In essence, the method comprises forming a mixture of potassium hydroxide, ammonium molybdate and silica, adding to the mixture phosphoric acid, solutions in nitric acid of the nitrates of cobalt, iron, nickel and bismuth, and more silica to form a slurry, then spray drying and calcining to form the catalyst.
It will readily be appreciated that one of the by-products of this reaction sequence is a large amount of ammonium nitrate especially as molybdenum is usually the major component (in atomic terms) of the finished catalyst and large quantities of ammonium molybdate must therefore be used. This by-product remains in the mixture until the catalyst is subjected to a high temperature treatment at which point it is driven off in the form of ammonia, water vapor and nitrogen oxides. The ammonium nitrate elimination, besides being inconvenient from the point of view of control of the gases eliminated is also very time consuming.
It is found moreover that when placed in an ammoxidation reactor the newly formed catalyst promotes undesirable side reactions for an extended initial period. This undesirable behavior is characterized by an excessive amount of burn of the ammonia reactant, as much as 30-40% of the ammonia being lost in this way.
As a result the reactor has to be operated at somewhat less than peak efficiency until the catalyst has gone through this initial phase of its activity.
It has now been found that by preparing the catalyst in a particular novel way, the difficulties attendant on the ammonium nitrate elimination in the prior art process referred to above are avoided.
Moreover, the catalyst produced in addition to being ready for immediate use at optimum or close to optimum efficiency, has an advantage over catalysts with the same metal ratios but prepared by the prior art process when used in ammoxidation reactions in that it is possible to operate with much closer to the stoichiometric amounts of ammonia and olefin without producing troublesome amounts of by-products.
Additionally, it has been found that, when used to produce acrylonitrile from propylene by an ammoxidation process, the novel catalyst prepared by the process of the invention has demonstrated substantially better results in terms of selectivity to and yield of acrylonitrile over catalysts with the same metal ratios prepared by the prior art process referred to above.